This invention relates to boundary layer transition measurements and more particularly to a method of visualizing laminar to turbulent boundary layer transition in flight environments.
The visualization of laminar to turbulent boundary layer transition plays an important role in flight and wind tunnel aerodynamic testing of aircraft wing and body surfaces. Visualization helps provide a more complete understanding of both transition locations and transition modes. Without visualization, the transition process is very difficult to understand.
Several flow visualization methods have previously been developed to measure laminar boundary layer transition to turbulence. These methods include the use of oil, flows, liquid films, china clay and subliminating chemicals. These prior methods suffer from numerous disadvantages which preclude their practical use for in-flight aerodynamic testing. Chief among these disadvantages is that for many atmospheric conditions the previous methods are not of practical use, particularly at higher altitudes and colder temperatures. Moreover, these methods do not have a rapid time response: and, as the methods are not reversible, only one transition measurement per flight can be obtained. Liquid crystals have great potential for overcoming the debilitating limitations of prior methods of boundary layer flow visualization.
Liquid crystals are a peculiar state of matter between solid and liquid. Although they appear as oily liquids, they have certain mechanical properties which are similar to solid crystals. In particular, liquid crystals scatter light very selectively. In their molecular state they are arranged in a series of slabs. Each molecule is long (with an aspect ratio of about 8), and in each slab the molecules are aligned with each other. The slabs are rotated slightly, one on top of the other, forming a helix of molecular orientation. Fortuitously, the pitch length of the resulting helix is in the range of wavelengths of visible light. Thus, when the helix is subjected to certain physical influences, the helix pitch changes and the wavelength of reflected light changes accordingly. In this fashion liquid crystal coatings change colors in response to changes in shear stress, temperature, pressure, ferromagnetism, and certain chemical vapors. Since the fundamental chemical structure is unaffected by these changes, a liquid crystal coating will respond repeatably to the same physical changes. Thus, the color changes of some liquid crystal formulations can be reversible virtually indefinitely.
Accordingly, it is an object of this invention to provide a new method of in-flight boundary layer transition visualization which is reversible, non-toxic, easy to apply, inexpensive, and has a rapid time response.
A further object of this invention is to provide a method of in-flight transition visualization which is applicable throughout the altitude and speed ranges of all subsonic aircraft flight envelopes.
A further object of this invention is to provide a method of boundary layer transition visualization which is applicable to water tunnel research.
Other objects and advantages of this invention will become apparent hereinafter in the specification which follows.